Various manufacturing facilities, such as petroleum refineries, chemical production plants, and the like, may generate a significant amount of vapors that are contaminated with hydrocarbons, volatile organic compounds, and the like, that are typically treated and expelled through various types of discharge vents.
Many conventional systems and methods used in petroleum refineries, chemical production plants, and the like, typically rely on granular activated carbon (GAC) systems and methods to remove the contaminants from the vapors that are expelled through the discharge vents and/or process sewers. The process typically involves directing the contaminated vapors to two vessels, connected in series as lead and lag vessels, each having granular activated carbon (GAC) therein. The contaminated vapors adsorb to the GAC to provide treated vapors which are discharged to the atmosphere. The discharged vapors must meet strict federal regulatory compliance guidelines, e.g., provided by the Benzene Waste Operations National Emission Standards for Hazardous Air Pollutants (NESHAPs), also known as “BWON.”
However, such conventional systems and methods require frequent and expensive replacement of the GAC in the treatment vessels. This results in process interruption during the replacement process, logistical complexities associated with moving the vessels throughout the manufacturing facilities, potential operator error during vessel exchange, safety concerns, potential for carbon bed fires, liability concerns associated with the waste byproducts from the regeneration of the GAC, and environmental concerns such as inconsistent regulatory compliance.
Some conventional systems and methods which utilize GAC to remove contaminated vapors may use heat exchangers to dehumidify the flow of contaminated vapors. This is done because GAC can be adversely impacted by humidity. Heat exchangers are also sometimes used to prevent bed fires, hydrocarbon polymerization, and/or oxidation of some solvents to toxic or insoluble compounds. See e.g., Khan et al., “Removal of Volatile Organic Compounds From Polluted Air”, Journal of Loss Prevention in the Process Industries, 13 (2000), 527-545, and Muzenda, E., “A Critical Discussion of Volatile Organic Compounds Recovery Techniques”, International Journal of Biological, Ecological and Environmental Sciences Vol. 2, No. 4, 2013, 73-78, both incorporated by reference herein.
One significant challenge of systems and methods which use GAC for treating contaminated vapors is the inability to effectively regenerate the spent or used GAC on-site at the facility. Conventional systems and methods which attempt to regenerate the spent GAC on-site provide incomplete regeneration because they may not be able to fully desorb (e.g., remove) the contaminants from the spent GAC. This results in a phenomenon known as “heel development,” and the spent GAC must be transported off site to a thermal regeneration facility, where the carbon is heated to about 1,500° F. to about 1,700° F. thereby destroying the adsorbed contaminants.
The challenge of heel development may be overcome by using synthetic adsorptive media in place of GAC in the vessels. Examples of synthetic adsorptive media include, inter alia, polymeric resins, such as DOWEX OPTIPORE® V503 and carbonaceous resins, such as AMBERSORB® 560. These resins may be regenerated in place, on-site, in the vessels using steam from the facility and may be reused, typically hundreds or thousands of times, without significant heel development and without the need move or transport the synthetic adsorptive media or the vessel(s).
The disadvantages associated with synthetic adsorptive media include: it is more expensive than GAC and typically has a lower adsorptive capacity for the hydrocarbon contaminants at petroleum refineries, chemical production facilities, and the like. The advantages of synthetic adsorptive media include: it can be regenerated on-site and in place in the vessels using sources of steam from the facility and typically does not have any problems associated with heel development and therefore can be regenerated many times without significant loss of capacity. Moreover, synthetic adsorptive media is not adversely impacted by humidity. Thus, a contaminated vapor stream from the regeneration process does not require dehumidification prior to treatment, which may be needed by some conventional GAC systems and methods.